1. Field of the Invention
This invention relates to a growth method of a nitride III-V compound semiconductor, manufacturing method of a semiconductor device, and semiconductor device which are especially suitable for application to semiconductor lasers, light emitting diodes or electron transport devices using nitride III-V compound semiconductors.
2. Description of the Related Art
GaN compound semiconductors are direct transitional semiconductors having forbidden band widths ranging from 1.9 eV to 6.2 eV and enabling realization of light emitting devices capable of emitting light over a wide range from the visible spectrum to the ultraviolet. For these properties, they have lately become of major interest and placed under active developments. Additionally, GaN semiconductors have a large possibility as material of electron transport devices such as FET. Saturation electron velocity of GaN is approximately 2.5xc3x97107cm/s, which is larger than those of Si, GaAs and SiC, and its breakdown electric field is as large as approximately 5xc3x97106 V/cm next to diamond. For these reasons, GaN semiconductors have been expected to be greatly hopeful as materials of high-frequency, high-temperature, high-power electron transport devices.
These semiconductor devices, in general, are made of GaN semiconductors grown on a substrate. Therefore, crystalline qualities of GaN semiconductors are of great importance for ensuring and improving performances of these semiconductor devices. However, since there is no appropriate substrate good in lattice matching with GaN, sapphire substrates are mainly being used for growing GaN semiconductors, their lattice mismatching with GaN is very large.
Failure in lattice matching with a substrate largely affects crystalline properties of GaN compound semiconductors grown thereon, and it can be a large factor of crystal defects produced in GaN semiconductor layers.
To minimize crystal defects, conventionally employed was a technique of growing a buffer layer of GaN or AlN on a sapphire substrate under a low temperature, then increasing the substrate temperature to about 1000xc2x0 C. to re-crystallize it, and thereafter growing GaN semiconductors thereon, thereby to improve the quality of GaN semiconductors (for example, Appl. Phys. Lett. 48(1986)353, Jpn. J. Appl. Phys. 30(1991)L1705).
Even with this technique, however, reduction of crystal defects is limited, and the density of defects (especially, threaded dislocation) is still as high as 108 through 1010 cmxe2x88x922.
For the purpose of decreasing the density of such defects, it has been reported to grow a GaN layer on a substrate conventionally used to grow III-V compound semiconductor like GaAs conventionally, then provide a mask of an insulating film such as silicon oxide film in form of elongated belts extending in the  less than 11-20 greater than  direction in predetermined intervals on the GaN layer, and thereafter selectively grow a GaN layer by hydride vapor phase epitaxy (HVPE) (for example, Jpn. J. Appl. Phys. 36(1997)L899). This technique can certainly decrease the density of threading dislocation to approximately 6xc3x97107cmxe2x88x922.
There are also other techniques of performing selective growth on a substrate via a mask made on the substrate to extend in a direction different by 90xc2x0 from that of the above-introduced example, and making a semiconductor light emitting structure on the selectively grown film. This technique is configured to grow a GaN layer on a sapphire substrate, for example, by metal organic chemical vapor deposition (MOCVD), next make a mask of silicon oxide to extend in the  less than 1-100 greater than  direction in form of elongated belts in predetermined intervals, thereafter grow a GaN layer thereon by MOCVD and further make a light emitting structure (Appl. Phys. Lett. 72(1998)211, Jpn. J. Appl. Phys. 36(1997)L899). According to these reports, the density of threading dislocation can be decreased to approximately 1xc3x97107 cmxe2x88x922. In these examples, it has been confirmed that the lifetime of a semiconductor laser made above is elongated to 1000 hours or more.
However, according to the Inventor""s own knowledge, the GaN semiconductor layer obtained by conventional growth methods introduced above still include a lot of crystal defects near the boundary with the substrate, and the density of defects has not been reduced sufficiently.
It is therefore an object of the invention to provide a growth method of a nitride III-V compound semiconductor capable of growing a single-crystal nitride III-V compound semiconductor of a high quality with a low density of crystal defects, and also relates to a semiconductor device manufactured by using this growth method and a manufacturing method of the semiconductor device.
The inventor conducted research to overcome the problems involved in the conventional techniques. The inventor""s research is summarized below.
FIGS. 1 and 2 show results of measurement of X-ray diffraction spectrum, taking samples each prepared by growing a GaN layer by MOCVD on a c-plane sapphire substrate via a mask of SiO2 made in form of elongated belts extending in the  less than 1-100 greater than  direction in predetermined intervals on the c-plane sapphire substrate, and introducing X-rays into one of the samples from a direction horizontal to the mask (see FIG. 3) and into the other sample from a direction vertical to the mask (see FIG. 4).
It is confirmed from FIGS. 1 and 2 that, although the c-axis inclination exhibits a single-peak property in the samples in which the X-rays enter in parallel to the mask, it exhibits a multi-peak property in the samples in which X-rays enter vertically in the mask. It has been noted through Transmission Electron Microscopic (TEM) analysis that the longitudinal crystal axes deviate at three positions on areas of the mask for selective growth and areas without the mask as shown in FIG. 5. In FIG. 5, inclination of the crystal axes on the mask is not limited to the illustrated example.
If a selectively grown film includes any inclination in crystal axis, especially if it includes discontinuous changes as indicated above, then it is presumed that lattice defects such as dislocation have been introduced along the boundary. Actually, dislocation was observed through TEM, and introduction of such defects may be a factor of deteriorating characteristics of a semiconductor laser made thereon.
The Inventor found, as a result of various researches, that the use of a nitride to form the outermost surface of the mask be effective to prevent inclination of crystal axes of a film grown on the mask for selective growth.
FIGS. 6 and 7 show results of measurement of X-ray diffraction spectrums, taking samples each prepared by growing a GaN layer by MOCVD on a c-plane sapphire substrate via a SiN/SiO2 mask made by stacking a SiN film on a SiO2 film in form of elongated belts extending in the  less than 1-100 greater than  direction in predetermined intervals on the c-plane sapphire substrate, and introducing X-rays into one of the samples from a direction horizontal to the mask and into the other sample from a direction vertical of the mask.
It is confirmed from FIGS. 6 and 7 that, by selective growth using the mask whose outermost surface is SiN, whichever of the parallel and vertical directions of the mask the X-rays are introduced in, each sample exhibits only one as the peak indicating inclination of crystal axes. Additionally, it is confirmed that the full width half maximum indicating variance in inclination of crystal axes within the measured range. This demonstrates a high crystalline quality of the selectively grown film.
This does not mean that crystal axes of the selectively grown film change in every region as shown in FIG. 5, but does demonstrate that longitudinal crystal axes are aligned over the entirety of the selectively grown film as shown in FIG. 8 and that the entirety of the film is uniform in quality.
Especially in a semiconductor light emitting device using a nitride III-V compound semiconductor, if the mask used for selective growth is a multi-layered film using silicon nitride or titanium nitride as the nitride forming the outermost surface of the mask and using titanium as the underlying layer, selective growth is easier than the case using titanium alone as the mask for selective growth, because the more stable nitride forms the surface. Additionally, when titanium is used as the n-side electrode, the current in an n-type layer, which could conventionally flow solely in the transverse direction, can readily flow in the longitudinal direction. Therefore, its operation voltage can be reduced.
It is confirmed from FIGS. 7B and 7C that, even in selective growth using a mask with the outermost surface of TiN similarly to selective growth using the mask with outermost surface of SiN, each sample exhibits a single peak, which indicates inclination of crystal eyes, both in the parallel and vertical directions relative to the mask. Additionally, it is confirmed that the full width at half maximum (FWHM) indicating variance in inclination of crystal axes within the measured range. This demonstrates a high crystalline quality of the selectively grown film.
The present invention has been made through these researches by the Inventor.
According to the first aspect of the invention, there is provided a growth method of a nitride III-V compound semiconductor for forming a growth mask on a substrate and selectively growing a nitride III-V compound semiconductor on the substrate by using the growth mask, characterized in:
using as the growth mask a multi-layered film in which at least a top surface thereof is made of a nitride.
According to the second aspect of the invention, there is provided a manufacturing method of a semiconductor device for forming a growth mask on a substrate, and selectively growing nitride III-V compound semiconductors on the substrate by using the growth mask, characterized in:
using as the growth mask a multi-layered film in which at least a top surface thereof is made of a nitride.
According to the third aspect of the invention, there is provided a semiconductor device using nitride III-V compound semiconductors, characterized in that a growth mask, in which at least a top surface thereof is made of a nitride, is formed on a substrate, and nitride III-V compound semiconductors are selectively grown on the substrate by using the growth mask.
In the present invention, the nitride on the surface of the multi-layered film forming the growth mask may be any nitride basically. However, its specific examples are silicon nitride (SiN) and titanium nitride (TiN). Thickness of the nitride is preferably in the range from 1 nm to 3 xcexcm. The multi-layered film may be any of various combinations such as combination of an oxide film and a nitride film thereon, combination of a metal film and a nitride film thereon, combination of an oxide film, a film thereon made up of a nitride and an oxide, and a nitride film thereon, and combination of a first metal film, a second metal film thereon different from the first metal film and a nitride film thereon, for example. The oxide film may be a silicon oxide film, for example, the nitride film may be a metal nitride film such as silicon nitride (SiN) film or titanium nitride (TiN) film, the metal film may be a titanium (Ti) film or a platinum (Pt) film, and the film made up of a nitride and an oxide may be gradually a silicon nitride oxide (SiN1xe2x88x92xOx (where 0 less than x less than 1)) film, for example. Any of these multi-layered films may be gradually changed in composition at one or more boundaries, if so desired.
Configuration of the growth mask may be selected appropriately from various configurations. A typical configuration thereof is a stripe extending in one direction relative to the substrate.
In the present invention, after forming on the substrate a first growth mask in form of a multi-layered film including a nitride to form at least its top surface, then selectively growing a first nitride III-V compound semiconductor on the substrate by using the first growth mask, and thereafter forming a second growth mask in form of a multi-layered film including a nitride to form its top surface on the first nitride III-V compound semiconductor above the locations of the substrate not covered by the first growth mask, a second nitride III-V compound semiconductor may be selectively grown on the first nitride III-V compound semiconductor by using the second growth mask.
In the present invention, the growth mask may be used as an electrode. In this case, the growth mask must include a conductive film such as metal film as its bottom layer, or include a conductive nitride as its top surface. Specific examples of the growth mask are combination of a metal film and a nitride film thereon, combination of a metal film, a film thereon made up of a nitride and an oxide, and a nitride film thereon, combination of a first metal film, a second metal film thereon different from the first metal film, and a nitride film thereon, combination of an oxide film and a titanium nitride film thereon, combination of a metal film and a titanium nitride film thereon, combination of an oxide film, a film thereon made up of a nitride and an oxide, and a titanium nitride film thereon, and combination of a first metal film, a second metal film thereon different from the first metal film, and a titanium nitride film thereon. The same films as those mentioned above are usable as the oxide film, the metal film, and the film made up of a nitride and an oxide.
In the present invention, the substrate may be a sapphire substrate, SiC substrate, Si substrate or spinel substrate, with or without a nitride III-V compound semiconductor grown thereon.
In the present invention, usable for growth of the nitride III-V compound semiconductor are metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), are molecular beam epitaxy (MBE).
In the present invention, the nitride III-V compound semiconductor is made up of at least one group III element selected from the group consisting of Ga, Al, In, B and Tl, and one or more group V elements which include at least N and may additionally include As or P, where appropriate. Specific examples of the nitride III-V compound semiconductor are GaN, AlGaN, AlN, GaInN, AlGaInN and InN.
In the present invention having the above-summarized structure, since a multi-layered film, which includes a nitride forming at least its top surface, is used as the growth mask, the nitride on the top surface brings longitudinal crystal axes of the grown film into alignment during selective growth of the nitride III-V compound semiconductor, and thereby reduces irregular alignment of longitudinal crystal axes of the grown layer.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.